A. Field of the Invention
This invention pertains to a novel apparatus and method for diagnosing the heart condition of a patient by monitoring his pulmonary function.
B. Description of the Prior Art
Studies have demonstrated heart failures are related to one or more of the following conditions: coronary artery disease, valvular heart disease, heart muscle disease, hypertension, pericardial disease, congenital heart disease, infection, tachyarrhythmia, and cardiac tumors.
Fundamentally, aerobic exercise training may decrease the symptoms associated with congestive heart failure (CHF) and increase heart rate variability (HRV), indicating an improved autonomic responsiveness and improving a patient""s exercise tolerance and quality of life. Current standard techniques for treating heart failure typically include angiotensin converting enzyme (ACE) inhibitors, diuretics, and digitalis. The main function of ACE inhibitors is to act as vasodilators, to reduce arterial blood pressure and reduce pulmonary capillary wedge pressure. ACE inhibitors have also been shown to increase cardiac output and increase the capability of heart failure patients to exercise. Diuretics decrease body fluid and hence reduce the cardiac load. Digitalis has been shown to increase the inotropic action of the heart and increase cardiac output in heart failure patients.
Ultimately, heart transplants and other cardiac surgeries are also alternative therapies, but only for a small percentage of patients with particular etiologies.
The New York Heart Association has developed a heart condition classification expressed as Stages I, II, III, and IV, determined as a combination of symptoms exhibited by a patient, including the ability to breathe properly, exercise or perform normal physical activities . Stage I is characterized by no breathlessness with normal physical activity. Stage II is characterized by breathlessness associated with normal physical activity. Stage III is characterized by breathlessness associated with even minimal physical activity. Stage IV is characterized by breathlessness even while a patient is at rest.
Several methods for monitoring the progression and severity of CHF are known. One powerful non-invasive method of stratifying mortality comprises measuring the HRV. It is not, however, an effective means of predicting CHF and, specifically, cannot be used in patients having sick sinus syndrome.
Another method of tracking the progression of CHF includes measurement of the ejection infraction using echocardiography or catheterization. The ejection infraction is a measure of the cardiac systolic function and can be used as a predictor of mortality. The ejection infraction, however, currently cannot be measured conveniently, or continuously, in an ambulatory patient.
It is an objective of the present invention to provide a system for determining a patient""s breathing pattern and then to use this pattern to indicate a patient""s cardiac condition.
A further objective is to provide a cardiac treatment system which monitors a patient""s cardiac condition over a predetermined time period and, based on his breathing pattern, indicates whether the patient""s condition has improved, worsened or remaining unchanged.
A further objective is to provide a reliable technique for monitoring and tracking the severity of CHF in an ambulatory patient through the variability in his respiration rate and pattern.
Other objectives and advantages of the invention will become apparent from the following description of the invention.
CHF patients typically have a characteristic breathing pattern which is dominated by a low-frequency variation as compared to healthy persons. More specifically, CHF patients have a periodic breathing pattern characterized by sequential deep and shallow breaths (FIGS. 8B and 8C) as compared to the breathing pattern of a healthy person (FIGS. 9B and 9C). As illustrated in the figures, healthy persons have a regular breathing pattern with a peak frequency greater than 0.15 Hz. On the other hand, CHF patients tend to have an abnormal breathing pattern with a low frequency dominant signal (less than 0.03 Hz) resembling Cheyne-Stokes respiration, characterized by periods of respiration followed by periods of non-breathing. Abnormal breathing patterns may result in a gas exchange deficiency, autonomic nervous system disruption and/or other problems. FIGS. 8A-8C and 9A-9C further illustrate a correlation in both the time and the frequency domains between the heart rate of a person and his respiration rate.
The automatic respiration mechanism, which is governed by the central nervous system and other peripheral functions, controls the respiration and heart rates, as well as the sympathetic and the parasympathetic cardiac efferent neurons. Conversely, the performance of the autonomic nervous system as well as the patient""s cardiac system can be derived from the variability of the respiration rate.
The present invention contemplates monitoring and recording the variability of respiration patterns for a cardiac patient. A respiration parameter (the respiration rate, tidal volume, low frequency power, high-frequency power, or ratio of the latter two) is first determined using an implanted device such as a defibrillator, a pacemaker or other event monitor. Currently, as these devices do not have sufficient calculating power to make the trend calculations necessary to determine respiration variability, the respiration parameter may be transmitted to an external device for processing. It is anticipated that future implantable devices will have sufficient calculating power to determine the respiration variability. The respiration parameter is then measured using the electrogram sensing interval or internal or external thoracic impedance. This respiration parameter is used to calculate the respiration variability and to indicate cardiac condition.
Continuously tracking respiration variability and breathing patterns of cardiac patients is beneficial for many other reasons as well. First, a clinician may track the progression or reversion of cardiac disease. Second, as certain landmarks are reached, the clinician could administer treatment or change therapies proactively. Third, the respiration variability provides a physician with an indication of how well a recovering cardiac patient is responding to a prescribed rehabilitation program.